This invention concerns a novel process for the conversion of sulfur dioxide to elemental sulfur.
Sulfur dioxide is found in the vent gases from industrial plants involved in the roasting, smelting and sintering of sulfide ores as well as those from power and industrial plants which burn sulfur-bearing fuels such as coal and oil. The desire to prevent both environmental air pollution and the loss of significant sulfur values has resulted in considerable effort to remove the sulfur dioxide from these vent gases. Since elemental sulfur represents a stable and easily transportable form, much of this effort has been directed to the reduction of sulfur dioxide to sulfur, either directly from the vent gases or indirectly from regenerative sulfur dioxide absorption systems.
Attempts at the reduction of sulfur dioxide to sulfur have included numerous approaches. The reduction can be accomplished by the Claus reaction, but this approach requires the handling and transport of toxic and flammable hydrogen sulfide. Alternatively, a thermal or catalytic reduction with a reducing gas such as hydrogen, carbon monoxide or a low molecular weight hydrocarbon may be employed, but such reducing agents are relatively costly.
A more recent approach to the reduction has therefore been with the use of the relatively abundant and inexpensive hydrocarbon coal. Such a reduction is the so-called Resox process disclosed, for example, in U.S. Pat. Nos. 4,147,762 and 4,207,292 and in which a stream containing sulfur dioxide is contacted with a moving bed of particulate coal at a temperature of from 1150.degree. to 1550.degree. F. (621.degree.to 843.degree. C.) in the presence of steam. While this process is relatively inexpensive and nonpolluting, it does possess certain disadvantages, which include the tendency of the coal particles to agglomerate at the reaction temperatures employed; the inherent large excess of coal in the reaction zone with resultant low effective utilization of the coal and potential contamination of the product sulfur with coal impurities such as tars; and the formation of by-product hydrogen sulfide and carbonyl sulfide.
Attempts to overcome such drawbacks are disclosed in British Patent Specification No. 1,552,886, in which a fluidized bed comprised of coal and a solid diluent is used to minimize bed agglomeration, and in Japanese Patent Application Disclosure 59604/81, in which a second reactor is employed to convert the hydrogen sulfide and carbonyl sulfide in the exit stream of the primary reactor to sulfur by low temperature reaction with a portion of the original sulfur dioxide laden stream and thereby improve the coal utilization.
It is the primary objective of the present invention to provide an alternative and simple process for the conversion of sulfur dioxide to elemental sulfur using coal as the reductant which results in a minimum formation of hydrogen sulfide and carbonyl sulfide and a maximum utilization of the coal.
Plasma arc methods have been employed for effecting various chemical reactions at high temperature. Such reactions include the conversion of carbon dioxide to carbon monoxide with carbon as disclosed in U.S. Pat. No. 4,190,636 and the reaction of coal with hydrogen to form acetylene and of carbon with nitrogen to form, cyanogen mentioned by D. M. Considine in Chemical and Process Technology Encyclopedia, b 1974, page 880.The reduction of sulfur dioxide, however, has not been suggested.